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Grounding Aluminum Gas Tanks
Posted: Fri Apr 25, 2008 10:56 am
When installing an aluminum gas tank, do you need to electrically ground the tank? If so, what is the most common way to do so?
Posted: Fri Apr 25, 2008 11:48 am
YES! All components of the fuel system must be grounded, Including the filler. The ground goes to your "earth" ground. This is to dispel static and prevent sparks.
Posted: Fri Apr 25, 2008 12:27 pm
How do most people attach the wires to the tank? and what guage wire should be used?
Posted: Fri Apr 25, 2008 12:44 pm
Most tanks have a tab with a hole in it for a bolt. The wire size is not critical. It is just to drain static electricity.
Posted: Fri Apr 25, 2008 1:00 pm
I've seen a wire stripped back & led up over the edge of the hose & down into the hose before it's attached to the filler neck to ground the filler.
You could also use one bolt & nut when attaching the filler to the deck with the wire attached to the underside.Then that wire goes to the tank ground tab & the earth ground.
Tank will have to be grounded anyway for the gauge sender to work
Aluminum Tank Installation
Posted: Fri Apr 25, 2008 2:38 pm
I hope that the (static bond wire) wire wasn't copper laid directly to the aluminum as this can make a battery by the difference in the two metals.
I think good practice would be a separate (grounding/bonding/earth/engine/hull common and [battery neg. too should be in this bond]) tab welded to the tank with a copper conductor attached. How the copper wire is attached is important, in my opinion, to avoid a corroded joint and high resistance in the bond. I've always used silver soldered tinned lugs, but a small piece or SS tube crushed into a cable/conductor as a lug works well too. This allows the copper to silver soldered in the SS and then there's little or no galvanic difference to the aluminum tank's bond tab.
The deck fitting should have an external bond to the tank, wrapping around the fill rise hose is pretty common. The tank should be bonded/common to the battery neg of the entire system and that should be bonded to the engine also.
All senders on metal tanks should have two wires and a dielectric (plastic gasket and washers) mounting kit to insure there is no charge on the tank from the sender.
If you use automotive style (one wire) senders and use the tank as DC return (neg.) the tank will be an anode and you're essentially dissolving the tank at the threaded fittings using the battery via the sending unit. If the tank were damp and charged then there will be external corrosion too, single wire senders are not best practice.
Its also critical to keep the tanks surfaces dry/drained/air exposed. Foam bedding destroys more unpainted tanks than any other factor. Urethane foam breaks down into formic acid and corrodes aluminum fast.
Posted: Fri Apr 25, 2008 5:41 pm
This is what I did for my tank installation following CFR 183, ABYC, and the Tank MFG ideas:
The tank is mounted on a full contact ply board sealed with 3 coats of epoxy, and mounted above wet line in bilge.
The tank has a coal tar epoxy coating.
Neoprine strips 4" wide (1/4" thick) are glued to bottom of tank at the baffles and each end. Glued with 3M 5200 spread with a putty knife to get 100% contact.
Tank bolted down with SS bolts, nylon washers and dielectric compound.
Two wire sender with plastic gasket, washers and compound.
10 Ga. high strand tinned copper marine wire crimpped in tinned lug with compound and heat shrunk tubing attached to welded lug on tank. Other ends same treatment and bolted to deck fitting and ground lug.
With tinned wire and tinned lugs the only copper exposed is the end of the strands where the are cut, this is sealed with the compound.
I think it is very important to get and use the proper marine tools and materials when wireing a boat. I know the tools are costly, but the correct cutter leaves very clean ends, the correct crimping tool gives the best crimp, etc. The cost of copper has made this real painful but, the best wire will pay off many times over.
I should say that I run in fresh water. At 430 engine hrs. I have found no corrosion anywhere in my setup.
Ship Shape Install
Posted: Fri Apr 25, 2008 5:57 pm
Nicely done, 50+. If a tank is worth mounting its worth mounting well, and sounds like you did.
Posted: Fri Apr 25, 2008 8:01 pm
Welcome. I've been sitting back on your post, watching for a sales pitch. But, you've come in with really good information and help. Thanks for joining in.
Posted: Fri Apr 25, 2008 9:20 pm
I built welded aluminum boats during the 70's and 80's in South Central Alaska, and moved to another type of business after a few hundred aluminum fishing boats and less than a dozen rec. boats in all those years.
No sales, just interest in metal boats from anyone's drawing table - but I'd like them to be built as well as we their builders can. I currently tutor a dozen builders on four continents and some of my neighbors, no costs, just help avoiding the millions of mistake I made trying to learn to build a metal boat that lasted and always got my skipper and his crew home.
I still design and build for enjoyment but not for sale!
I drop by here, from time to time, to kibitz on metal boats, and may have a bit to say about welding too. Old welders are all full of... opinions.
Thanks, Bill, for the welcome.
Back to port
Posted: Sat Apr 26, 2008 7:02 am
I like your idea of building boats that last. Fifty Plus is probably my last big project in my life, and useing it for many years and always getting back to the dock is what it is all about!
PS Thanks for the input.
Re: Grounding Aluminum Gas Tanks
Posted: Wed Dec 06, 2017 8:07 am
Any suggestions on how to mount an aluminum tank to a fiberglass hull? There are no galvanic concerns of course but it seems like I still want to keep water from accumulating under the tank and corroding it. I like the idea of full contact rubber strips and 3M 5200 adhesive to elevate the tank from the below deck ~1/4".
The full contact ply board that Fifty plus mentioned seems like overkill in my application though.
I plan to use fiberglass mat and epoxy to bond the tabs of the tank to the fuberglass hull.
A ground will be attached from the tank to the negative buss bar of the battery to provide a ESD Drain.
Re: Grounding Aluminum Gas Tanks
Posted: Wed Dec 06, 2017 3:16 pm
I'm not a fiberglass experienced builder but I have built and mounted my share of welded aluminum and SS tanks for various purposes including fuel oils and gasoline. So my remarks are coming from the metal side of the discussion not necessarily from an informed fiberglass builder. (grain of salt and all)
First question I'd ask is the size of the tank- if the tank will be heavy/large then I'd suggest designing and welding on engine mount style mounting fixtures that bolted to glassed in engine type stringers. On the other hand if the tank is not so heavy as to require engine-mass-sized mounting? then bedding and straps can work too.
My cut off of large versus small tank is based on the wt of liquid and tank; combined. An example: a 35 gallon tank will carry 220lb of gasoline (approx numbers only) and take up 4.6 cubic feet of volume so the sides might be 2' x 2-4" top and bottom and (2 sides) @ 1' x 2' and (2 sides) @ 1' x 2'4" of plate- 1/8 will work for this so about 25 feet squared of tank sheet, including a couple full size baffles. That's about 1.728 lb per ft^2 or 44 lb (almost all the figures are rounded up). That means the tank and contents as gasoline will come to about 250 -270lb. depending on the fittings, mounting hardware, fill hose and the other unlisted items.
That is a lot of wt to be mounted in a hull and deserves plenty of respect in the form of very secure mounting. So- if the tank is that large (35gal) or larger I'd recommend an engine mount style installation with matching fittings to cover a pair of glassed in stringers/frames or other 'hard points' in the structure on which to base the mounting.
Next, if the tank is smaller than 35gal. , then the wt isn't as critical an effect on the hull framing- (my arbitrary cut off of 250lb as "critical" is just an opinion- not written in stone anywhere) mounting using an isolation bedding material is probably reasonable. Even in the type of install where an entire tank side is the 'base' or mounting surface it is important to plan straps, tabs to structure or some other restraint to keep the tank where its mounted even in heavy pitching or rolling motions.
[I think we've had at least some discussion on mounting metal tanks on a tank thread in the "Metal" category of the Index page? May have some useful ideas to review?]
Bedding a tank on the bottom side is a mixed blessing - reducing the need for a fully rigid tank that is essentially hanging or suspending the contents in the structure of the tank -like and engine is suspended on beds/stringers/hull longitudinals. Using one side of the tank to support the wt on a hull surface- or on dedicated mounting platform- allows the metal to be in direct contact with glass, perhaps elastomeric bedding products, other metal parts even on the entire bottom surface of the tank.
That kind of installation is possible to contribute to both crevice corrosion and galvanic corrosion if the bilge water is allowed to slosh onto a bronze bilge pump or through hull and then wet the tank?
So if the tank is smaller, and will be bedded onto the bottom surface- keeping that surface dry or sealed from water intrusion is pretty important. Lots of installers will insist that aluminum tanks are acid etched, primer painted and then top coated with an epoxy of coal tar type of paint to keep the aluminum surface free of contaminates that may lead to corrosion.
As to the idea of covering the tank mount tabs with glass and resin- I'd ask if you have some method of sealing the 'enclosure'? What I'm getting at is the metal will be covered with resin and cloth but.... the tank side of that welded on tab will have an unsealed edge-opening-exposure to the bilge volume. That would be a concern for me this is setting up a crevice corrosion cell- a close fit area, where water can get in- even atmospheric humidity is sufficient to start a corrosion cell but that water has little or no reason to 'dry out' the cavity of the glass to aluminum tab surface.
The idea of keeping the tank surface above the surrounding surfaces using some rubber/plastic strip is valid; but make sure you look up the actual material's reaction to aluminum? Some carbon based rubber will actually form a galvanic cell with aluminum, I found this out by using an old dive suit's neoprene to bed a tank once!! the tank rotted along the bedding strips! the spongy material glued to the base plate/mount and the tank- small straps were used to keep the tank on the glued in strips (but in less than 10 years that tank's bottom showed leaks!) and I was totally mystified until I found an article mentioning the potential for some bedding material to react galvanically- another reason many builders want paint systems on their aluminum tanks.
Bonding the tank, and all metal fittings connected to the DC, NEG buss is good practice it will help to eliminate any static build up but also any tank level electrical gauge's stray current if there were any present?
Last remark is that if you're using SS fasteners to hold the tank mounts, straps, tabs or any other part- I'd like to encourage you to make sure they're passivated to reduce as much as possible the galvanic interaction with aluminum. Simply buy the SS passivization acid gel and soak the parts to improve all SS's resistance to galvanic action with aluminum. I've found that hot dipped galvanized fasteners are about as long lasting as any other metal used to mount aluminum because Zinc and Aluminum are so close on the galvanic scale.
Hope others with more experience will take a minute to post in reply, in the meantime- I hope you're getting some ideas to explore and research to find an answer to your questions?
Re: Grounding Aluminum Gas Tanks
Posted: Wed Dec 06, 2017 3:28 pm
Lots of good info & experience in Kevin's post... avoiding galvanic reaction at support points is essential.
BTW here's a how-to article on the topic by Dave Pascoe from a while back; still useful I think:
Re: Grounding Aluminum Gas Tanks
Posted: Wed Dec 06, 2017 3:59 pm
Steve, thanks for the article link. I'd defer to D.Pascoe in about every aspect of marine surveying as he's the "Pro from Dover" and does it for a living, has written more books than I can count with my shoes on, and is generally an authority on the subject.
The only clarification I'd make is he calls for an avoidance of galvanized (zinc coated) on aluminum and that is inaccurate in my experience. There is a difference between fittings/plumbing/pipe threads and fasteners and that is at the heart of our remarks.
Fittings like pipe nipples, King nipples and other liquid conveyance sealed connections to an aluminum tank- will be hard to use in galvanized. The reason is that zinc plating of hot-dipped galvanized fittings is essentially very 'rough' at the microscopic level. So zinc/galvanized fittings will tear up tender and very soft aluminum female pipe threads- and I agree with Mr. P in regard avoiding these fittings in a NPT combination.
HOWever, fasteners- bolts and nuts ONLY (not tapped threads) zinc coating is a very good choice as the galvanic scale potential is less than SS, even when properly passivated, so I'd separate the two zinc plated ( but tank related) sets of hardware when reading D.Pascoe's recommendation to avoid galvanized on tanks.
Great link, very informed and educated in aluminum surveyor.